An optical input device that uses the self-mixing effect is described in U.S. Pat. No. 6,707,027, which has been filed in the name of Applicant. This document explains the measuring principle and describes several embodiments and applications of the sensor device, which may be included in a computer mouse, for example.
Laser self-mixing occurs if an external reflector, or object, is arranged in front of a diode laser so that an external cavity is obtained. In case of an input device, movement of the device and the object, which may be a human finger or a desk surface, with respect to each other causes tuning of the external cavity. Such tuning results in re-adjustment of the laser equilibrium conditions and thus in detectable changes in the laser output power. These changes, or undulations, are repetitive as a function of the displacement of the object over a distance equal to half the wavelength of the laser radiation. This means that that the frequency of the laser undulations becomes proportional to the speed of the object, i.e. the external reflector. A measuring device based on laser self-mixing shows high sensitivity and accuracy, which can be attributed to the fact that the back-scattered radiation re-entering the laser cavity determines the modulation frequency of the laser radiation and thus is amplified in the laser cavity. The laser acts as a phase sensitive detector and amplifier. In this way high receiver sensitivity is obtained without using additional means like optical filters or complex devices, such as interferometers.
An optical sensor module of this type equipped with two diode lasers allows measurement of movements of the input device, including such a module, and the object with respect to each other in two mutually perpendicular (X- and Y-) directions and any intermediate direction. Although such module can already detect motion in the Z-direction, it is preferred to use a 3rd diode laser, as a module having three diode lasers allow a more accurate detection of motion in the Z-direction. This can be achieved by mutually arranging the laser beams such that they are orthogonal or such that they make a 120 degree angle in the detection plane. Such an input device can be used to navigate, or move, a cursor across a display panel, for example to select an icon on the display.
Preferably, the diode laser used in the present input device is a vertical cavity surface emitting laser (VCSEL). The length direction of the laser cavity and the radiation direction of such a laser are perpendicular to its substrate. Compared with the more conventional edge-emitting laser, a VCSEL shows the advantages that it can be fabricated at much smaller size and lower costs and that the cross section of the laser beam is circular, instead of oblong. Moreover the self-mixing effect is considerably stronger in this type of laser.
The concept of the above-described input device has proven to be very worthwhile and an optical computer mouse equipped with such a device has been successfully manufactured and introduced on the market. However, this computer mouse is of the high-end type. In view of its compactness, accuracy and reliability the optical sensor module is also very suitable for low-end computer mice and other mass applications. For such applications the costs of the optical sensor module becomes a main issue in view of the prices of existing devices having lower performance. Thus there is a strong need to reduce the cost of the optical sensor module considerably. Since the number of main elements of the module can not be reduced, cost reduction can only be realised by means of a cheaper manufacturing process.